This study is designed to evaluate the therapeutic effects of thymopentin in patients with Mycosis Fungoides and Sezary Syndrome. Thymopentin is a pentapeptide derived from the thymic hormone thymopoietin, which has demonstrated activity as an immunostimulant in patients with autoimmune disease, immune deficiencies, and HIV infections and has been shown to induce both complete and partial remissions in patients with the Sezary Syndrome. In this study, Thymopentin is administered at a dose of 50 mg in 50 cc IV over 10 minutes, t.i.w. (Mon., Wed., Fri.) for 16 weeks. Side effects on this study have been minimal and include fever, chills and headache. There have been six patients entered on study; among three evaluable patients, two developed progressive disease and one had a partial response.